In general, an organic electroluminescence element (an organic EL element) has a fundamental layer structure formed with an anode, an organic light emission layer, and a cathode, and is completed by adding layers such as a hole injection layer, a hole transport layer, an electron transport layer, and an electron injection layer to the layer structure as appropriate.
In an organic EL element having such a structure, it is known that there is normally a trade-off relationship between luminous current efficiency and operating life. So as to improve the trade-off relationship, various studies have been made on organic materials, organic layer structures, element drive methods, and the like.
From the viewpoint of an organic layer structure, a so-called tandem structure has been suggested as a technique for fundamentally improving the trade-off relationship. In a tandem structure, a group of luminescent functional layers formed with one or more layers including a light emission layer made of at least an organic material is regarded as one light emitting unit, and such light emitting units are stacked via an intermediate layer, so that the operating life can be prolonged while luminescent current efficiency is maintained, or luminescent current efficiency is dramatically increased without substantially shortening the operating life.
Intermediate layers in such tandem structures are disclosed in Patent Literatures 1 to 7 described below.
“Patent Literature 1” discloses a structure that has two conductive layers between OLED layers, and specifically discloses a stack structure formed with a Mg—Ag alloy film and an ITO film, and the like.
“Patent Literature 2” discloses an intermediate conductive layer formed with at least two layers, and specifically discloses a stack structure formed with an 8-nm thick Mg—Ag alloy film and a 10-nm thick In—Zn—O conductive film, and the like.
“Patent Literature 3” discloses a technique for achieving conductivity by doping a floating thin conductor layer containing an organic compound with an acceptor or a donor.
“Patent Literature 4” and “Patent Literature 5” also disclose intermediate layers formed by stacking organic layers doped with an acceptor and a donor.
“Patent Literature 6” discloses a structure in which a metal layer (an internal electrode) with a work function of 4.5 eV or lower is inserted between light emitting units, and “Patent Literature 7” discloses a similar structure in the description of embodiments.
However, the demand for increases in the performance and the productivity of an organic EL element is becoming more and more difficult.
For example, in the structures disclosed in “Patent Literature 1” and “Patent Literature 2”, two conductive layers needs to be stacked, and the film formation process differs from the film formation process for an organic EL element normally formed by a vapor deposition method or a coating method, since the ITO film and the In—Zn—O film disclosed as embodiments need to be formed by a sputtering method. For these reasons, the load is too large in terms of production efficiency. Also, in a case where such conductive layers are formed on an organic material by a sputtering method, damage to the organic film cannot be ignored.
With the methods using doped layers described in “Patent Literature 3”, “Patent Literature 4”, and “Patent Literature 5”, the process load can be made smaller than that in “Patent Literature 1” and “Patent Literature 2”. However, performance varies depending on doping density, and therefore, it is difficult to secure production stability. Further, there is a high risk of performance degradation due to diffusion of a doping material into the respective layers constituting the light emitting units. Therefore, such organic EL elements in use for many years have a possibility of accelerated performance degradation.
A structure in which one metal layer is inserted between light emitting units as described in “Patent Literature 6” and “Patent Literature 7” has relatively low process load, and have a low possibility of performance variation due to material diffusion, compared with the structures disclosed in “Patent Literature 1” and “Patent Literature 2”.
Specifically, although a 6-nm thick structure using Al (work function: approximately 4.3 eV) is disclosed in the description of embodiments in “Patent Literature 6”, the drive voltage remains high only with the Al layer, and an electron injection layer adjacent to the anode side of the metal layer becomes necessary. A structure in which LiF is used as an electron injection layer material is actually disclosed. As a result, one inorganic layer needs to be inserted, and the production load increases. Also, it is considered that the two-layer structure formed with LiF and Al substantially realizes the functions of an intermediate layer. Furthermore, the metal layer is thick, having a thickness of 6 nm. Therefore, preservation stability and drive stability are not ideal, and there is also decrease in efficiency due to absorption by the metal layer.
A structure using a 0.3-nm thick Li layer as a metal layer (an intermediate unit) is disclosed in the description of embodiments in “Patent Literature 7”. However, the thickness of the metal layer is only approximately 0.3 nm. Therefore, it has become clear that the organic EL element does not exhibit stable performance, or there is large performance variation particularly in the initial stage of operation or during a relatively short period of time after the preparation of the element.